https://xmpp.org/extensions/xep-0077.html#registrar-formtypes defines 
 a form that can be used in any context (ad hoc, etc) with standardized 
 names for the common things one might want at registration, including 
 password.
 
 
Thanks, I missed that. I now see the form type is managed on the 
registrar: 
https://xmpp.org/registrar/formtypes.html#jabber:iq:register
XEP-0389 mentions a FORM_TYPE of urn:xmpp:register:0 in the examples but 
there doesn't seem to be any plan of actually registering such a form 
type. Would it make sense to mention the existing jabber:iq:register 
namespace in an example (either existing or a new one) since that's 
likely to be a common use case? Perhaps one using jabber:iq:register 
fields plus a custom field as that would serve as a complete example.
Of course, the real answer would have to been properly read up on field 
standardisation, but this could provide a hint for those not familiar 
with the specs. I'd be happy to write such an example, if we decide to 
include it.